For quite some time reactive polyurethane compositions have been known which can be applied in liquid form and which are used as coatings that provide bridging over cracks in order to seal construction work in respect of ingress of water; they are also termed liquid membranes. In comparison with prefabricated polymer webs, they provide easier application, in particular when surfaces are uneven or have complex geometry, they improve protection from below-surface migration by virtue of adhesion to the substrate across the entire area, and they also provide seamless laying. In comparison with non-reactive systems applied in liquid form, for example polymer solutions, aqueous polymer dispersions, or bitumen-based products, they feature high strength and good resilience, even at low temperatures, have low susceptibility to soiling, and provide durable sealing even under standing water.
The properties of a hardened liquid membrane in providing bridging over cracks over a wide temperature range are important for reliable prevention of water ingress into construction work. In order to achieve this, the hardened material should have high extensibility, high strength, and good tear strength. A rather low value for modulus of elasticity is advantageous here, in order that movements caused by way of example by temperature variation or vibrations in the construction work do not give rise to stresses in the membrane that are excessive and that can cause separation of the membrane from the substrate or cohesive fracture within the substrate. The latter can be especially critical in the case of substrates having low resistance to pressure, an example being insulation foam.
The expression polyurethane liquid membranes covers not only one-component systems but also two-component systems. Curing of one-component polyurethane liquid membranes is brought about by moisture, and takes place from the outside toward the inside, and these membranes therefore require a relatively long time for complete curing throughout the material, in particular when thick layers are used, the environment is dry, or there is restricted availability of moisture; this can impact on subsequent operations. Commercially available products moreover include considerable content of volatile organic solvents, firstly in order to increase shelf life, and secondly in order to reduce viscosity and thus improve ease of use. For this reason, they emit VOC, discernible by their strong odor, and they exhibit a degree of shrinkage.
In contrast to this, two-component polyurethane liquid membranes, even those with high solids content, have comparatively low viscosity, and are therefore easy to apply; they also cure rapidly throughout the material, even when the materials are relatively thick. However, during hardening they are very sensitive to moisture and temperature. If humidity is high, in particular when this is combined with high temperatures, or the substrate is damp, or there is direct contact with water, evolution of CO2 can form bubbles; this leads to foaming of the coating, and impairs its sealing function and robustness; in warm conditions they have a short open time, and at low temperatures they cure very slowly or remain soft and tacky. Furthermore, many known two-component products include solvents and/or volatile isocyanates, and can impact on protection of the environment and prevention of health hazards.